1. Field of the Invention
The present invention relates to a liquid crystal display device with a transverse electric field type, and more particularly, to a large screen active matrix liquid crystal display device with low power and cost as well as improved display quality such as a large visual field angle and high contrast and brightness.
2. Description of the Related Art
With the use of a pair of comb-like electrodes formed on one of the substrates, an active matrix liquid crystal display device in a transverse electric field system is achieved where an electric field is applied to a liquid crystal composition layer on the substrate in a horizontal direction. Such a technology has been proposed, for example, in Japanese Laid-Open Patent Publication Nos. 7-36058 and 7-159786. FIGS. 1 and 8 are plan views of the conventional liquid crystal display device in the transverse electric field system.
In the example of FIGS. 1 and 8, each pixel includes a pixel electrode which is formed of a pair of a common electrode 3 and a liquid crystal drive electrode 6 for driving the liquid crystal and a scanning line 1. FIG. 2 shows a drive signal waveform for driving the liquid crystal display panel in the conventional transverse electric field system. The voltage of the common electrode (Vcom) is constant, and with reference to the common electrode voltage (Vcom) as a center, a video signal 8 changes its polarity at every horizontal scanning cycle.
As shown in FIGS. 1 and 8, when improving an aperture ratio of the liquid crystal panel as well as the transmission of a backlight therethrough, a distance between a common electrode 3 and a liquid crystal drive electrode 6 in each pixel has to be increased. When the distance between the two electrodes becomes large, a voltage for driving the liquid crystal needs to be increased to accommodate the distance. In the conventional drive method where the constant common electrode voltage (Vcom) is used, the amplitude (VDH+VDL) of the video signal voltage applied to a video signal line 2 needs to be increased to about 20V (volts). In this situation, the maximum voltage amplitude (VGH+VGL) of a scanning signal 7 applied to the scanning line 1 is about 50V. Further, due to the increase in these drive voltages, the cost of a driver IC has to be significantly increased.
Moreover, the increase in the drive voltages tends to increase short circuits in the scanning lines 1 and the video signal lines 2. Also, when the signal voltage of video signal line 2 increases, the shielding effect of the common electrode 3 declines, resulting in vertical cross talks. The voltage increase further leads to the increase in the electric field applied between the pixel electrode (common electrode and liquid crystal drive electrode) and a transparent electrode which is formed on the entire backside of a color filter substrate, allowing ions to move into the liquid crystal layer. Such ions cause problems such as residual images, resulting in serious deterioration in the display quality.